CD Wow are selling "Mario Kart DS" and "New Super Mario Bros" for the Nintendo DS for £19.99 each (inc Del.) But with the attached voucher, you can get them for £18.99.
The next best in-stock price seems to be £23.79 for Kart (dvd.co.uk) and £21.49 for Bros (ChoicesUK)
To get these prices:
1. Follow the link to the attached voucher thread.
2: Click the "Use Voucher" button. (This will take you to CD-Wow)
3: Search for "Mario Kart" and/or "Super Mario Brothers"
4: The discount should automatically be applied so just add them to your cart and pay for them as normal. :-)